Electrochemical plating methods

ABSTRACT

An electrochemical process for applying a conductive film onto a substrate having a seed layer includes placing the substrate into contact with an electrochemical plating bath containing cobalt or nickel, with the plating bath having pH of 4.0 to 9.0. Electric current is conducted through the bath to the substrate. The cobalt or nickel ions in the bath deposit onto the seed layer. The plating bath may contain cobalt chloride and glycine. The electric current may range from 1-50 milli-ampere per square cm. After completion of the electrochemical process, the substrate may be removed from the plating bath, rinsed and dried, and then annealed at a temperature of 200 to 400 C to improve the material properties and reduce seam line defects. The plating and anneal process may be performed through multiple cycles.

PRIORITY CLAIM

This application is a Divisional of U.S. patent application Ser. No.14/219,940, filed Mar. 19, 2014, now pending and incorporated herein byreference.

FIELD OF THE INVENTION

The field of the invention is methods for electrochemically processingmicro-scale work pieces, wafers or substrates.

BACKGROUND OF THE INVENTION

Microelectronic devices, such as micro-scale electronic,electro-mechanical or optical devices are generally fabricated on and/orin work pieces or substrates, such as silicon wafers. In a typicalfabrication process, for example on a semiconductor material wafer, aconductive seed layer is first applied onto the surface of the substrateusing chemical vapor deposition (CVD), physical vapor deposition (PVD),electro less plating processes, or other suitable methods. After formingthe seed layer, a blanket layer or patterned layer of metal is platedonto the substrate by applying an appropriate electrical potentialbetween the seed layer and one or more electrodes in the presence of anelectro processing solution containing metal ions. The substrate is thencleaned, etched and/or annealed in subsequent procedures, to formdevices, contacts or conductive lines. Some substrates may have abarrier layer with the seed layer formed on the barrier layer.

Currently, most micro-electronic devices are made on substrates platedwith copper. Although copper has high conductivity, it typicallyrequires a barrier layer such as tantalum nitride (TaN) to preventdiffusion of copper into the substrate or dielectric material on thesubstrate. These types of barrier layer have relatively lowconductivity. Using known techniques, features on the substrate arefilled with electroplated copper using acid copper chemistries orelectroplating solutions. These chemistries often use additives topromote a super conformal fill process (with the features fillingprimarily from the bottom up, rather than inwardly from the sides) andcreate a void-free fill. As the feature sizes shrink, achieving voidfree fill with the traditional copper plating processes has become moredifficult. Also as the features get smaller, the barrier layer requiredfor copper occupies a larger volume, because a minimum barrier layerthickness must be maintained to prevent copper diffusion, regardless offeature size.

For example if a minimum barrier layer thickness of 3 nm is required toprevent diffusion of copper, then for a feature having a 60 nm criticaldimension with an aspect ratio of 4:1, the barrier layer occupiesroughly 11% of the cross-sectional area. However, with a feature ahaving a 20 nm critical dimension with an aspect ratio of 2:1, thebarrier layer must remain 3 nm thick, but it now occupies 33% of thecross sectional area. In this case the volume of the barrier layer(which has low conductivity) is proportionally higher, so the resistanceof the interconnect, via or other feature is proportionally higher. Withprogressively smaller features, the proportion of copper to barrierlayer increases, to the extent that the resistance becomes unacceptable.

One approach proposed for overcoming this technical challenge is toreplace copper with a metal that does not require a barrier layer, suchas cobalt. Although cobalt has a higher resistance than copper (6uOhm-cm for cobalt versus 2 uOhm-cm for copper), cobalt does not requirea barrier layer because it does not diffuse into the silicon ordielectric. U.S. Patent Application Publication No. 20130260555describes filling large and small features by applying cobalt viachemical vapor deposition (CVD). Although this method works well forsmaller features, e.g., of 7-10 nm, CVD is not well suited for fillingfeatures larger than about 10 nm. Improved techniques are needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a scanning electron microscope (SEM) image of an unfilled orunplated structure on a substrate having a TaN barrier layer and a Cuseed layer. In some applications the barrier layer may be othermaterials, such as TiN, or there may be no barrier layer. The seed layermay also be CVD cobalt.

FIG. 1B shows the structure of FIG. 1 now filled with electrochemicallydeposited cobalt from an alkaline plating bath.

FIG. 2A is a SEM image of a similar structure electroplated with cobalton a TaN/Co seed layer using a cobalt-glycine plating bath having a pHof 6.5, with seam line defects showing.

FIG. 2B shows the structure of FIG. 2A after annealing, with the seamline minimized or eliminated.

FIG. 3 is SEM image of another structure electroplated with cobalt on aCVD Co seed layer using a cobalt-EDA plating bath having a pH of 8.5.

FIGS. 4A-4E are schematic diagrams of an embodiment of the presentmethods.

FIG. 5 is a graph of test data of line resistance after annealing.

FIGS. 6A-6C are schematic diagrams of super conformal fill.

FIGS. 7A-7C are schematic diagrams of conformal fill.

FIGS. 8A-8C schematic diagrams of conformal fill followed by annealing.

DETAILED DESCRIPTION

Various known cobalt plating methods using acid and alkaline cobaltbaths have been proposed. See for example U.S. Patent ApplicationPublication No. 2014-0008812. However, plating cobalt onto substrateshaving very small features, for example features of 60 nm, 40 nm, 30 nmor less, presents different challenges. Substrates with very smallfeatures necessarily have a very thin seed layer. Using known cobaltplating methods on these substrates will usually dissolve the very thinseed layer, preventing proper plating. The present methods use a cobaltbath with a specific pH range to minimize corrosion of the seed layer.

Nickel has plating characteristics similar to cobalt. The described usesof cobalt may be applied as well to use of nickel instead of cobalt.References here to interconnects includes other features used on or insubstrates, such as trenches, holes and vias.

Deposition of a metal inside a sub-micron interconnect may be achievedby electrochemical deposition on a conductive substrate. The platedmetal can be selected from a list including copper, cobalt, nickel,gold, silver or platinum. Conformal and super conformal electrochemicaldeposition of the metal may be followed by an optional thermaltreatment.

A neutral to alkaline aqueous solution may be used for deposition of theelectrochemically plated metal. For example, cobalt or nickel complexplating solutions may be used to electrochemically deposit cobalt ornickel into sub-micron interconnects or other features on a substrate.The substrate may be provided with a seed layer formed via electro lessdeposition, physical vapor deposition, or chemical vapor deposition.Materials used in the seed layer may include copper, manganese dopedcopper, ruthenium (Ru), and others. Cobalt silicide or nickel silicidemay also be used in the seed layer. The barrier layer on the substrate,if any, may be applied via chemical vapor deposition (CVD) or usingother known techniques.

The electroplating or electrochemical deposition process may be followedby an annealing step to improve the material properties of theelectrochemically plated cobalt or nickel, and to reduce seam line voidsassociated with conformal electroplating.

In the present methods, annealing after plating may be performed attemperatures lower than used for traditional copper processes. Theanneal step stabilizes the plated film. It may also help remove seamlines and micro voids from the conformal plating process. The annealstep may also improve film properties by driving out impurities that canbe trapped due to the plating conditions. With some applications,depending on specific plating conditions and chemistries, the annealstep may be omitted. For example a cobalt plating solution that promotessuper conformal growth and incorporates low impurities may not need ananneal step.

A. Electrochemical Deposition Using Cobalt i. Overview

Methods of the invention are diagrammatically shown in FIGS. 4A-4E. InFIG. 4A, a substrate 10, such as a silicon wafer has features 12 and aconductive seed layer 14. A barrier layer (not shown) may be providedunder the seed layer 14 in some applications. In FIG. 4B, a cobalt ornickel conformal or super conformal film 16 is plated onto the seedlayer 14. The film 16 may partially or fully fill the features, withFIG. 4B showing the film 16 partially filling the features 12. Thethickness of the film 16 is sufficient to provide an at least 10, 20,30, 40 or 50% fill of the features (in contrast to the seed layer 14which provides virtually no significant fill). The barrier layer mayoptionally be PVD TaN, ALD TaN, PVD TiN, ALD TiN, ALD MnN, CVD MnN, CVDNiSi or CoSi, where PVD is physical vapor deposition, CVD is chemicalvapor deposition and ALD is atomic layer deposition.

FIG. 4C shows annealing with the film 16 reflowing to further fill thefeatures 12. FIG. 4D shows deposition of a capping or metallizationlayer 18, which may be the same metal (cobalt or nickel as used for thefilm 16), or a different metal. FIG. 4E shows the substrate afterchemical mechanical polishing, with the capping layer 18 selectivelyremoved leaving filled features 20.

The film 16 may be electro plated onto the seed layer 14 using a neutralto alkaline cobalt plating solution ranging from pH 4 to pH 9. Theplating solution may contain a chelating agent such as citrate, glycine,tartrate, ethylene diamine, etc.

FIGS. 6A-6C illustrate super conformal filling of a feature on asubstrate, such as a trench or via. FIG. 6A shows the seed layer. FIG.6B shows a partial fill with super conformal ECD. FIG. 6C shows a seamfree fill with super conformal ECD. As shown, the feature largely fillsup from the bottom, rather than inwardly from the sides, providing aseam-free plated feature. Filling may also be performed by plating aconformal film followed by an annealing step, or by another layer ofsuper conformal film. FIGS. 7A-7C show conformal filling where thefeature is largely filled inwardly from the sides, with a seam in thefilled feature, similar to FIGS. 6A-6C. FIGS. 8A-8C show the sameprocess as in FIGS. 7A-7C, but with the filled feature seam-free afterannealing, with FIG. 8A showing the seed layer, FIG. 8B showing fullfill with conformal ECD, and FIG. 8C showing a seam-free fill afterannealing. Conformal or super conformal plating may be used inperforming the described methods.

ii. Detailed Process Description

1. The substrate is provided with a conductive seed layer such as CVD orelectro-less cobalt, although others such as copper, nickel, gold,silver, palladium and/or ruthenium may be used. FIG. 1A shows an exampleof an unfilled feature with a barrier layer such as TaN applied onto thesubstrate and a copper seed layer on the barrier layer. FIG. 2A showsconformal electrochemical deposition (ECD) of cobalt on substrate havinga cobalt CVD layer on a TaN barrier, with seam line defects apparent.FIG. 3 shows an example of conformal ECD cobalt on a CVD cobalt seedlayer.

2. A pre-plating treatment may be used, i.e., reducing agents such asHe/H2, forming gases, etc. may be applied to the substrate, beforeplating. The structure may be electroplated with cobalt in a platingbath that is mildly acidic, neutral or basic. In the examples of FIGS. 1b, 2 a, and 2 b, a cobalt chloride and glycine bath at pH 6.5 was usedfor cobalt deposition. In FIG. 3,. a cobalt chloride and EDA bath at pH8.5 was used for cobalt deposition. The current density used for theelectrochemical deposition process may range from 1-50 milli-ampere persquare cm.

3. A neutral to alkaline plating solution may be used when the seedlayer is more susceptible to corrosion, such as with CVD cobalt seedlayers. Full coverage of electrochemical deposition of cobalt on acobalt seed layer applied via chemical vapor deposition may generally beobtained when the pH is increased from 6.5 to 8.3. The plating bath mayalternatively have a pH in one of the following ranges: 7.5 to 8.5; 7.8to 8.5; 8.0 to 8.5; or 7.8 to 9.0.

4. After conformal or super conformal electrochemical deposition ofcobalt is completed, the substrate may be thermally treated attemperatures of 200 C to 450 C to improve the material properties and/orreduce seam line defects. FIG. 2 b shows a substrate after annealing at350 C in a H2/He environment. The seam line is no longer visible in thescanning electron microscope image. Other gases such as N2/He or pure H2may alternatively be used. Surface roughness is also improved after theannealing process as shown in FIG. 2 b.

A multi plate multi anneal process may be performed by filling thefeatures with a slow plating process, then annealing to improve thematerial properties, followed by depositing the capping layer 18 forchemical mechanical polishing. Plated cobalt may be used for the cappinglayer 18. In a multi plate process having first and second plating stepsproviding first and second films on the substrate, after annealing thesubstrate, for example at a temperature of 200-450 C, a third platingstep may be performed to provide a metallization layer on the secondfilm. The metallization layer may then be chemically mechanicallypolished.

iii. Test Results

Features ranging 60 nm-25 nm have been filled using the methodsdescribed. Test results show successful plating on thin seed layershaving a high sheet resistance, i.e., on 200 ohm/sq seed layer on 300 mmwafers. This type of seed layer, which would typically rapidly corrodein a conventional acid copper plating solution, is not significantlyetched or corroded using the cobalt or nickel plating solutionsdescribed above. Test results also demonstrate successful plating of acobalt film on a 6 nm CVD cobalt seed layer, using a mildly acid toalkaline cobalt plating solution.

Test data also show a decrease in line resistance and blanket filmresistance with anneal treatment after plating, as shown in FIG. 5. Amulti plate multi anneal process on substrates having a CVD cobalt seedlayer has also been performed. One example of a multi plating process isto fill the features with a slow plating solution, and then move thesubstrate to another chamber for a fast plating of the cap ormetallization layer 18, in advance of chemical mechanical polishing.

B. Electrochemical Deposition Using Nickel

The methods and parameters described above may also be used with nickel.

In contrast to a CVD only process, the methods described above providefor much higher through put and decreased cost, so that they are welldesigned for high volume manufacturing.

Thus, novel methods have been shown and described. Various changes andsubstitutions may of course be made with departing from the spirit andscope of the invention. The invention, therefore, should not be limited,except by the following claims and their equivalents.

1. An electrochemical process for applying a conductive film onto asubstrate having a seed layer, comprising: placing the substrate intocontact with an electrochemical plating bath containing cobalt or nickelsalts, with the plating bath having pH of 4.0 to 9.5; conductingelectric current through the bath to reduce cobalt or nickel ions in theplating bath and deposit a super conformal film of cobalt or nickel ontothe seed layer.
 2. The method of claim 1 wherein the seed layercomprises copper, cobalt, gold, silver, nickel, palladium and/orruthenium on the substrate.
 3. The method of claim 1 with the platingbath comprising cobalt chloride or other cobalt salts.
 4. The method ofclaim 1 with the plating bath comprising glycine or another chelatingagent.
 5. The method of claim 1 with the electric current ranging from1-50 milli-ampere per square cm.
 6. The method of claim 1 furtherincluding annealing the substrate at a temperature of 200 to 450 C. 7.The method of claim 6 with the annealing causing the film to reflow. 8.The method of claim 6 wherein the annealing is performed in anenvironment of H2/He, N2/H2, a reducing gas, pure hydrogen, or ammonia.9. The method of claim 1 wherein the plating bath has a pH of 6 to 9.10. The method of claim 1 with the plating bath further including achelating agent which complexes cobalt metal ions.
 11. The method ofclaim 1 wherein the cobalt or nickel is deposited into sub-microninterconnects on the substrate.
 12. The method of claim 1 wherein theplating bath comprises a mildly acidic to basic cobalt or nickel platingsolution and includes a chelating agent selected from the group ofcitrate, glycine, tartrate and ethylene diamine.
 13. The method of claim1 with the seed layer on a barrier layer on the substrate.
 14. Themethod of claim 1 with the substrate having no barrier layer.
 15. Anelectrochemical process for depositing a conductive film into sub-microninterconnects on a substrate, with a seed layer on the substrate,comprising: placing the substrate into contact with an electrochemicalplating bath comprising a metal selected from cobalt or nickel, and withthe plating bath having pH of 4.0 to 9.0; conducting electric current ata rate of 1-50 milli-ampere per square cm through the bath and to thesubstrate to deposit a super conformal film of metal onto features ofthe substrate; and removing the substrate from the plating bath, rinsingand drying the substrate.
 16. The method of claim 15 with the platingbath comprising cobalt chloride.
 17. The method of claim 15 with theplating bath comprising glycine.
 18. The method of claim 15 furtherincluding repeating the steps at least once to provide a multi-stepmulti-cycle ECD and anneal process.
 19. The method of claim 18 furtherincluding annealing the substrate at a temperature of 200 to 450 C tocause the film of metal to reflow and further fill features of thesubstrate.
 20. An electrochemical process for applying a conductive filmonto a substrate having a seed layer, a first feature of a first sizeand a second feature of a second size greater than the first size,comprising: placing the substrate into contact with an electrochemicalplating bath containing cobalt or nickel salts, with the plating bathhaving pH of 4.0 to 9.5; performing a plating step by conductingelectric current through the bath to reduce cobalt or nickel ions in theplating bath and deposit a super conformal first film of cobalt ornickel onto the seed layer, with the first film entirely filling thefirst feature but not the second feature; performing a second platingstep by conducting electric current through the bath to reduce cobalt ornickel ions in the plating bath and deposit a super conformal secondfilm of cobalt or nickel onto first film, with the second film entirelyfilling the second feature.